1. Field of the Invention
The present invention relates to a hydraulic driving device for a working machine including an exhaust emission control device catching particulate matter in exhaust gas generated in an engine by a filter, making the exhaust gas carry heat required for burning the particulate matter by raising the engine output in a non-operation state of the working machine, and regenerating the filter by burning the particulate matter clogging the filter.
2. Description of the Related Art
A hydraulic driving device of a hydraulic excavator which is a kind of the working machine includes an engine, a main pump (variable displacement hydraulic pump) driven by being transmitted with power of the engine, a single rod double acting cylinder driven by discharge of the main pump, and a spool valve interposed between the main pump and a hydraulic actuator. The single rod double acting cylinder includes a rod chamber formed on a rod side of a piston and a bottom chamber formed on a bottom side of a cylinder tube, and is driven by being selectively supplied with oil discharged from the main pump to the rod chamber and the bottom chamber. The spool valve can switch to a first actuating position leading the oil discharged from the main pump to the rod chamber of the single rod double acting cylinder and leading the hydraulic fluid inside the bottom chamber to a hydraulic fluid tank, a second actuating position leading the discharged oil to the bottom chamber of the single rod double acting cylinder and leading the hydraulic fluid inside the rod chamber to the hydraulic fluid tank, and a neutral position leading the discharged oil to the hydraulic fluid tank without leading it to the single rod double acting cylinder.
In an exhaust pipe leading the exhaust gas of the engine to the outside of the working machine, an exhaust emission control device catching the particulate matter in the exhaust gas generated in the engine by a filter is provided. The particulate matter caught by the filter is removed from the filter by being burnt by the heat of the exhaust gas.
It is configured that, in a non-operation state of the hydraulic excavator, that is in a state the valve position of the spool valve is held at the neutral position, the engine output is lowered to the magnitude required for the variable displacement hydraulic pump to deliver the oil with the minimum discharge pressure and the minimum discharge amount required for cooling and lubricating a hydraulic circuit.
When the engine output drops, the temperature of the exhaust gas also drops. Accompanying it, the particulate matter is hardly burnt by the heat of the exhaust gas, and the filter of the exhaust emission control device comes to be liable to be clogged. In order to prevent clogging of the filter, a hydraulic driving device of a prior art is configured that, when clogging of the filter is detected, the discharge pressure of the main pump is raised, that is the load applied to the engine is increased, with the aim of raising the engine output and the temperature of the exhaust gas is raised to the temperature required for burning the particulate matter. (Refer to Japanese Patent No. 3073380 for example.)
When the discharge pressure of the main pump is raised in order to raise the temperature of the exhaust gas to the temperature required for burning the particulate matter, there is a concern that the discharge pressure of the main pump leaks from the spool valve and the discharge pressure is transmitted to the single rod double acting cylinder although the spool valve is in a state held at the neutral position.
In the single rod double acting cylinder, the pressure receiving area of the bottom side of the cylinder tube is larger than that of the rod side of the piston. Therefore, when the pressure of the same magnitude is applied to both of the pressure receiving surface on the rod side of the piston and the pressure receiving surface on the bottom side, the force pressing the piston from the bottom side is stronger than the force pressing the piston from the rod side. As a result, the piston is pressingly moved from the bottom side, and the single rod double acting cylinder expands. Accordingly, when the discharge pressure of the main pump leaked from the spool valve is transmitted into the rod chamber and into the bottom chamber of the single rod double acting cylinder, the single rod double acting cylinder may expand although the spool valve is held at the neutral position. The expansion of the single rod double acting cylinder is not an intention of an operator which is not desirable.